Week 6 has only one episode because that Sunday was Lunar New Year Eve, and Show was pre-empted for some specials (I think).
King Taejong has stepped down and Grand Prince Chung-nyeong ascends the throne, becoming who we’ll know as Sejong the Great. He sends Jang Yeong Shil and Jang Hee Jae with the Joseon envoys to Ming China, and has an important task for them – to sneak into a forbidden place which holds the legendary Chinese armillary sphere. Sejong hopes that Joseon can produce their own armillary sphere which is suitable for the Joseon skies, and never have to rely on Chinese almanac again.
But of course nothing is smooth sailing for Jang Yeong Shil, as he finds out belatedly the Chinese armillary sphere doesn’t exist anymore, and that crazed slave hunter official from Dongnae is now in Ming China and is hellbent on killing him. The eunuch whom Jang Hee Jae is supposed to seek help from was actually from Joseon, and while he pretends to agree to help Sejong, he also allies with that cruel official and tells Jang Hee Jae of the plan to kill off Jang Yeong Shil. Even though conflicted, Jang Hee Jae was blinded by his jealousy of Jang Yeong Shil’s talent and did nothing to stop Jang Yeong Shil from entering the forbidden place.
Despite always encountering danger, there’re always people who appreciate Jang Yeong Shil’s talent and take initiative to ensure his saftey. The royal Uncle of the Ming Emperor is one such person and upon witnessing Jang Yeong Shil’s talent, he and his daughter wanted to invite Jang Yeong Shil to their place to repair a replica of a huge water clock. They got wind of the plan to kill Jang Yeong Shil and saves him in the nick of time. Jang Yeong Shil is then convinced to repair the equipment and sets down to do what he loves most.
Upon failing to get rid of Jang Yeong Shil, Jang Hee Jae is next tasked by the eunuch to kill the royal Uncle. Instead of handing over the armillary sphere from Turkey that is laced with poison, Jang Hee Jae gets thrown to the enclosed area where Jang Yeong Shil is at, and gets beaten up by Jang Yeong Shil. Glancing at the water clock however, both the Jangs are bonded by not just their blood but also the love for physics, math and astronomy and put aside their difference to repair the clock together.
Eventually the ploy to kill royal Uncle is exposed, and Jang Hee Jae gets striped off his rank upon returning to Joseon. Sejong promised Jang Yeong Shil to elevate his status from a slave, and that decision meets with a whole load of disagreement. Sejong also secretly gathers some like-minded low ranking officials, especially those with interest in math and astronomy, and together with Jang Yeong Shil and Princess Sohyun, they secretly build the first armillary sphere of Joseon and write the Joseon almanac.